When you do three things at once, are you doing any of them well?
Seriously, stop and think about it. You're checking your email and your co-worker is asking you a question. Which one are you focused on? Maybe both, but then you're only giving each half your attention.
Computer science teaches that computers don't truly do three things at the same time. They do Task A, then jump to Task B, then Task C. Computers switch back and forth very rapidly, giving the illusion that they're multi tasking.
Most people don't realize this. Instead they see their computer running three apps at the same time and figure they can do, too. But we're not computers.
Pick one task and focus on it. Do it to the best of your ability. Focus is key, especially if it's a creative task or something you're still learning to do. Don't stop to answer the phone or check Twitter. Finish the task, or if it's a big job, take a break and walk around.
Consider quality over quantity. Doing something well versus doing ten things poorly. Now go try it.